A well-established body of literature supports the role of physical activity in maintaining mental health. Conversely, lack of physical activity has been associated with adverse mental health outcomes, such as posttraumatic stress disorder (PTSD) and high anxiety sensitivity. Emerging research has investigated and found promising support for aerobic exercise in reducing symptoms of PTSD and anxiety sensitivity through exposure to feared bodily sensations; however, little is known regarding whether factors such as cardiorespiratory fitness predict who will most benefit from such interventions. We explored the role of cardiorespiratory fitness in predicting reductions in PTSD symptoms and anxiety sensitivity following aerobic exercise. Thirty-two individuals from a previously published trial (Fetzner & Asmundson, 2015) were enrolled in a 2-week aerobic exercise program consisting of six sessions of moderate to vigorous intensity stationary cycling and were assessed for cardiorespiratory fitness. Results suggested that aerobic exercise was generally effective in reducing symptoms of PTSD and anxiety sensitivity, regardless of individuals' fitness levels; however, individuals with lower pre-treatment levels of cardiorespiratory fitness experienced greater reductions in avoidance and hyperarousal symptoms of PTSD, as well as total, physical, and social symptoms of anxiety sensitivity. With the exception of re-experiencing symptoms of PTSD and cognitive symptoms of anxiety sensitivity, symptom reduction generally decelerated or returned toward pre-treatment levels following post-treatment. Aerobic exercise may be particularly effective in targeting specific symptoms related to PTSD and anxiety sensitivity in individuals with lower levels of cardiorespiratory fitness and sustained physical activity may be required to maximize maintenance of treatment gains. (C) 2015 Elsevier Ltd. All rights reserved.